1. Field of the Invention
The present invention relates generally to the field of image registration and more specifically to the field of image registration configured for medical applications.
2. Related Art
Deformable image registration is a technique for modifying a deformable image in an elastic way to match similar features in a reference image. The technique, in general, involves determining a transform necessary to register (e.g., to align) two images by matching structures of interest in the deformable image and the reference image and to deform the deformable image to achieve alignment of these structures. The deformable and reference images may be images of a patient at different times. The deformable image may be an image of a healthy person and the reference image may be an image of a patient, or vice versa.
For example, the deformable image may include an image of a patient in which a physician has carefully delineated a treatment volume, and the reference image may include an image of the same patient recorded at a later time during treatment. A treatment volume is a spatial volume within a patient to be treated using radiation such as X-rays or particle beams. By deforming the carefully delineated image to align with an image recorded during treatment, movement in the treatment volume can be readily identified. The unaltered reference image, having a treatment volume identified through the image registration process, can then be used to target the new position of the treatment volume during treatment.
The technique has various medical applications. One medical application includes identification of movement or change in organs in medical images acquired at different points in time, for example, changes in patient anatomy since an image was taken at the outset of a treatment plan, for example, for planning purposes. Changes in organs can occur due to the spread of cancer, bladder fullness, swelling, breathing, etc.
Another medical application of the deformable image registration technique is segmentation of images using a pre-segmented atlas against which the images are registered. After registration is complete, an identity and/or scope of an anatomical structure within the reference image can be established automatically using previous identification of a corresponding structure within the deformable image. Both medical applications allow for the identification of, for example, changes in an organ during the course of treatment such that the treatment may be altered or discontinued.
Modification of the deformable image includes use of a deformation algorithm that includes a flexibility model of displacement vectors. Various deformation algorithms exist in the prior art, for example, a demons algorithm. The flexibility model in the demons algorithm comprises Gaussian smoothing of the field of displacement vectors. A “similarity” force is exerted on the displacement vectors by using a normalized optical flow. One advantage of the demons algorithm is the speed at which the steps of the algorithm are performed (e.g., on the order of minutes).
A disadvantage of existing deformation algorithms is that their flexibility model is constant across the field of displacement vectors. Certain structures in medical images require more flexibility than others. For example, during a breathing cycle, a lung slides against a membrane at an edge of the lung. Although the lung stretches in a smooth manner, the movement is difficult to account for using current models.
There is, therefore, a need for improved systems and methods of registering medical images.